In an image forming apparatus such as laser printers, there has been proposed a developing cartridge that is detachably mounted in a main body of the apparatus.
The developing cartridge has a box-shaped housing, for example. Toner is accommodated in the housing. A rectangular opening is formed on one side of the housing. A developing roller is rotatably supported by a first sidewall and a second sidewall, which face each other with the opening of the housing interposed therebetween. In other words, the developing roller has a developing roller shaft that extends in a longitudinal direction of the opening. Both end portions of the developing roller shaft are rotatably inserted in the first and second sidewalls, respectively, so that the developing roller is provided to be rotatable about the developing roller shaft. Outside the first sidewall, a developing gear is fixed to one end portion of the developing roller shaft at the first sidewall. When driving force is transmitted to the developing gear, the developing roller (developing roller shaft) is integrally rotated with the developing gear.
A gear cover is attached to an outer side face of the first sidewall. The developing gear and a plurality of gears, which form a gear train for transmitting the driving force to the developing gear, are collectively covered by the gear cover.